As you can see in the chart above, it's the first time since 2011 that more Americans said they'd prefer immigration decreased in the United States.

A statement from Gallup says:

Americans' views on immigration have varied a bit in the past 15 years, with the dominant view shifting between decreasing immigration and maintaining it at the current level. Some of these changes may reflect the ebb and flow of Americans' reactions to the 9/11 attacks in 2001 as well as rocketing unemployment in 2009, with both events triggering a temporary surge in anti-immigration sentiment.

Notice the other side of that -- the proportion of people who want increased levels of immigration have been steadily increasing since Gallup started asking this question back in 1999.

Gallup's also been asking people whether they believe that immigration is generally a good or bad thing, and there's been a slight recent decrease in the number of people saying it's a good thing:

"Immigration is central to who Americans are as a people, and what the United States represents, and by and large Americans view immigration as positive for the country," Gallup says in its explainer.
"But deciding how many new immigrants to welcome each year can be controversial, particularly when unemployment is high, and seeming competition for good jobs already fierce."

It's definitely not surprising that people with different political affiliations view the issue differently: